The digital subscriber loop interface circuit usually will comprise two parts; a transmitter and a receiver. The function of the transmitter is to put a series of pulses, usually shaped by some form of filter, on the loop. In the U-interface of an ISDN (Integrated Services Digital Network) system, these pulses are likely to be 4-level pulses, particularly encoded as the so-called 2B1Q code (two binary, one quaternary) recommended by the American National Standard Institute (Working Group T1E1).
The function of the receiver is to detect pulses being sent from the far end of the loop, which is difficult because these pulses are distorted such that relatively square pulses being transmitted from the far end are smeared by the time they arrive at the receiver. One source of distortion is coupling of the transmit pulses, being put onto the loop, directly across the hybrid circuit and into the receiver input as "echoes", which is a common problem when operating on a two-wire system.
Such transmit pulse echoes are removed by echo-cancellation, typically using a transversal filter to derive a function or model of the transmit signal for subtraction from the received signal. For an example of such an echo-canceller, the reader is directed to copending application Ser. No. 261,133, filed on Oct. 24, 1988 by Sami Aly et al, entitled "Apparatus and Method for Echo Cancellation", which is incorporated herein by reference.
Once the echo-canceller circuit has removed the image of the transmitter pulses from the received pulses, what remain are the pulses being transmitted from the far end of the loop. These pulses, of course, may still be distorted by intersymbol interference or ISI, which includes interference from symbols received before the symbol of interest as a result of variation of the loss and delay characteristics of the loop with frequency. As a result of the delaying characteristics of the loop, when symbols are transmitted, the tail of one symbol persists into the time slot of the next symbol, making it difficult to determine the correct amplitude of the pulse designated to that time slot. The pulses may also be distorted by added noise due to cross talk from signals transmitting along adjacent pairs. This might be near end cross talk or impulse noise. Another source of distortion is added low frequency noise due to electrical power lines.
Of particular relevance to the present invention, however, is post cursor distortion which results from the band-limited frequency response of the channel. In the time domain, this results in a received pulse response with a long tail. In addition this band-limited frequency response may produce precursor distortion which, in the time domain, shows as a received pulse response with a slow rise time that is not zero at any multiple of baud periods ahead of the sampling time instant. The problem is worse for full duplex transmission where, as mentioned previously, the local transmitted signal echo provides a significant distortion component usually necessitating echo cancellation.
These forms of distortion are interrelated so that a solution for one form can make the others worse. Moreover, the manner in which the distortion is treated can affect other functions in the receiver, especially timing recovery, which involves synchronizing the sampling clock to the received far end signal. For details of a timing recovery circuit suitable for ISDN, the reader is directed to copending application Ser. No. 07/261,134, now U.S. Pat. No. 4,896,334, entitled "Method and Apparatus for Timing Recovery" by Babak Sayar et al, which is incorporated herein by reference.
A digital data receiver usually will have an equalizer to correct the pulse distortion caused by the transmission channel. Post-cursor ISI is usually dealt with by means of a decision feedback equalizer (DFE). Although forward equalization also can eliminate, or at least reduce, the length of the tail, decision feedback equalization produces less noise enhancement and can be easier to implement. A disadvantage of decision feedback equalization is that errors are propagated, whereas forward equalization is less prone to error propagation and can take care of both pre- and post-cursor distortion.
If only a decision feedback equalizer is used, received pulses may be sampled at a point that will cause precursor ISI that cannot be cancelled by the decision feedback equalizer. In such a situation, the distortion might be cancelled using, say, sequence estimation methods such as proposed by W. Lee and F. Hill Jnr. in a paper entitled "A Maximum Likelihood Sequence Estimator with Decision Feedback Equalization", IEEE Transactions on Communications, Sept. 1977. In addition, a relatively more complicated timing estimate needs to be derived to sample the received pulses at the optimum sampling phase.
In order to address these problems it has been proposed to use a combination of a feedforward equalizer and a feedback equalizer. In this case a compromise has to be accepted between, on the one hand, the noise enhancement performance and, on the other hand, the complexity and convergence speed of the decision feedback equalizer.
Known equalizers use a low pass filter to limit high frequency noise. What follows the low pass filter will often differ according to whether the equalizer is generally analog or to be fabricated using DSP (digital signal processing) technology. An "analog" equalizer may use a simple DFE (decision feedback equalizer) in combination with an adaptive forward cable equalizer. The adaptive forward cable equalizer may be an ALBO (automatic line build out) equalizer, as proposed by Ephraim Arnon et al in a paper entitled "A Transmission System for ISDN Loops", IEEE 1986, or a .sqroot.f equalizer as disclosed by Misao Fukuda et al in a paper entitled "Digital Subscriber Loop Transmission Using Echo Canceller and Balancing Networks" IEEE August 1985; by Toshiro Suzuki et al in a paper entitled "Line Equalizer for a Digital Subscriber Loop Employing Switched Capacitor Technology", IEEE Trans. on Communications, Sept. 1982, or by Roy B. Blake, Noah L. Gottfried, B. J. Trivedi and William F. Zucker in a paper entitled "An ISDN 2B+D Basic Access Transmission System", AT&T Bell Laboratories, Whippany, New Jersey 07981, U.S.A.
Although the combination of a low pass filter and simple decision feedback equalizer lends itself to analog implementation, it has the disadvantage of enhancing high frequency noise and being too complex to implement digitally. With the increasing use of digital signal processing in data communications, it is desirable to implement the equalizer using a digital signal processor (DSP). When using a DSP, it may be preferable to use a fixed, simple forward equalizer but with a more complex DFE, typically with more taps than the relatively simple DFE used for the analog equalizer. The fixed forward equalizer may be a single-pole, single-zero equalizer such as disclosed by Michael Vry in a paper entitled "A New Transmission System for ISDN Access at 144 kB/s" ISSLS 1984, or a (1-Z.sup.-1) equalizer such as disclosed by P. F. Adams et al in a paper entitled "A Long Reach Digital Subscriber Loop Transceiver", British Telecom Technology Journal Volume 5 No. 1 Jan. 1987.
Although these techniques are more suitable for DSP implementation, they do not provide the degree of pulse shaping required substantially to minimize the precursor pulse distortion and the near end cross talk noise.